DAMASCUS, Syria — UN chemical weapons experts headed to a Damascus suburb on Wednesday for a new tour of areas struck by a purported poison gas attack, activists said, as Western powers laid the groundwork for a possible punitive strike and the UN chief pleaded for more time for diplomacy.

U.S. leaders, including Vice-President Joe Biden, have charged that Syrian President Bashar al-Assad’s government fired deadly chemical weapons near Damascus last week.

The U.S. has not presented concrete proof and UN inspectors have not endorsed the allegations.

Stephen Harper spoke with Obama on Tuesday about the escalating crisis his office later released a bluntly worded statement that said Canada is onside with its American ally as the U.S. contemplates military action.

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The UN’s envoy to Syria, Lakhdar Brahimi, said Wednesday that evidence suggests some kind of “substance” was used in the Aug. 21 attacks that, according to the group Doctors Without Borders, killed 355 people.

Syria, which sits on one of the world’s largest stockpiles of chemical weapons, has denied the charges.

Still, the U.S. and its allies appear to be moving toward a strike against the Assad regime, including by readying possible legal justifications.

Britain said it will put forward a resolution to the UN Security Council on Wednesday to condemn the Syrian government for the alleged attack.

Prime Minister David Cameron’s office said Britain would seek backing for “necessary measures to protect civilians” in Syria under Chapter 7 of the UN charter. Military force is one of the options that can be authorized under Chapter 7 of the UN charter.

Any language that could be read as paving the way for military strikes is likely to face veto by Damascus allies Russia and China.

A French diplomatic official said Britain’s decision to introduce the UN resolution was aimed at pressuring the Russians but has virtually no chance of passage. The official, who was not authorized to disclose details of the deliberations and spoke on condition of anonymity, called the resolution a “pretext” intended to demonstrate an attempt to go through the Security Council.

In Geneva, the UN’s Brahimi said any military strike must have UN Security Council approval.

Britain’s parliament is to hold an emergency session Thursday on a motion clearing the way for any British response.

As the political tension increased, so did uncertainty about what kind of role — if any — Canada would play in such a mission. Canada’s chief of defence staff, Gen. Tom Lawson, was in Jordan with military leaders from other western nations.

Opposition calls for Harper to recall Parliament to debate the crisis are growing louder, but government sources said that would only happen if Canada has a direct combat role.

Harper’s spokesman said Tuesday it is “premature” to decide whether MPs, who aren’t expected back in the House until October, would be recalled early.

In their phone call Tuesday morning, Harper and Obama were united in their condemnation of the use of chemical weapons by Syria.

Harper and Obama have instructed officials in their governments to “remain closely in touch in the days ahead,” said the Prime Minister’s Office.

Syrian activists have said that hundreds of people were killed in the poison gas attacks.

The UN chemical weapons experts conducted their first field testing in the western Damascus suburb of Moadamiyeh on Monday. They collected samples and testimony after a treacherous journey through government and rebel-held territory. Their convoy was hit by snipers but members of the team were unharmed.

On Wednesday, a convoy of UN cars left the Four Seasons Hotel in Damascus. Before their departure, the experts looked at maps and were briefed by UN disarmament chief Angela Kane and team leader Ake Sellstrom.

The team did not speak to reporters.

Three Syrian anti-regime activists, speaking on Skype, said the experts were in Eastern Ghouta, a large Damascus suburb affected by the alleged chemical attack. They spoke on condition of anonymity for fear of regime retribution.

The activists said the experts had crossed into rebel-held territory and were being accompanied by members of the Free Syrian Army, the main armed opposition group.

Two amateur videos posted online showed a UN convoy of five vehicles driving through what narrators said was the town of Mleeha in Eastern Ghouta. The UN vehicles were followed by several pickup trucks carrying armed men. The video corresponded with AP reporting of the events depicted.

UN chief Ban Ki-moon said the team has already “gathered valuable samples and interviewed victims and witnesses.”

Brahimi, the UN envoy, said that “it does seem like some kind of substance was used” in last week’s strikes.

“This was of course unacceptable,” Brahimi said. “This is outrageous. This confirms how dangerous the situation in Syria is and how important (it is) for the Syrians and the international community to really develop the political will to address this issue seriously, and look for a solution for it.”

The U.S., Britain and France haven’t shared with the UN any evidence they are collecting about the incident, he said.

Ban pleaded for more time to allow the UN team in Syria to establish the facts and to give diplomacy another chance to end the Syrian conflict, which has claimed more than 100,000 lives in two and a half years.

He said the Security Council, whose permanent members are bitterly divided over Syria, must not go “missing in action.”

Marking the centenary of a venue for peaceful conflict resolution at The Hague, Ban said: “Here in the Peace Palace, let us say: Give peace a chance. Give diplomacy a chance. Stop fighting and start talking.”

Two of Syria’s staunchest backers, Iran and Russia, warned of dire consequences if the U.S. and its allies attack in Syria.

Such strikes “will lead to the long-term destabilization of the situation in the country and the region,” said Russian Foreign Minister Sergey Lavrov.

Iranian Supreme Leader Ayatollah Ali Khamenei said Wednesday that attacking Syria would be catastrophic for the entire Middle East.

“Intervention of foreign and extra-regional powers in a country has no result other than sparking fire,” Iran’s state TV quoted Khamenei as saying. “Waging a war is like a spark in a gunpowder store … its dimensions and consequences can’t be predicted.”

In Israel, large crowds lined up at gas-mask distribution centres, amid speculation that Syria could hit Israel in response to any U.S. strike.

Maya Avishai of the Israeli postal service, which oversees gas mask distribution, said demand has tripled in recent days. About five million Israelis, roughly 60 per cent of the population, now have gas masks, she said.

In Jordan, government spokesman Mohammad Momani said the kingdom will not be a launching pad for attacks on Syria. Any Western attack on Syria would likely involve sea-launched cruise missile attacks.

Jordan is trying to avoid friction with its larger neighbour for fear the Assad regime or its Iranian backers could retaliate. Momani said Jordan prefers a diplomatic solution to the Syrian crisis.

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Laub reported from Beirut. Associated Press writers Yasmine Saker in Beirut, John Heilprin in Geneva, Jamal Halaby in Amman, Jordan, and Mike Corder at The Hague, Netherlands, and Jamey Keaten in Paris contributed reporting. With files from Postmedia